Microbial outgrowth is a primary concern in the food processing industry and amongst consumers. The presence of pathogenic microorganisms on food products can potentially lead to food-borne outbreaks of disease.
Chlorine-based chemicals such as sodium hypochlorite, calcium hypochlorite, sodium dichloroisocyanurate and quaternary ammonium compounds have been employed for disinfecting food products in the past. However, chlorine is most effective at a pH of 6 to 8, and becomes less effective outside of this pH range. Also, chlorine can produce toxic byproducts that are harmful to human health, such as chloramines and trihalomethanes.
As a result of this, the European Union has imposed a bar against the use of chlorine compounds for disinfecting food produce, as specified by the EU Directive 2092/91. There has consequently been a concerted effort to improve technology employing non-chlorine based products for the treatment of food products to disinfect them. This has resulted in an increased interest in the disinfecting properties of ozone. The use of ozone for disinfecting food has been approved by the United States Food and Drug Administration (FDA).
It is noted that ozone is reported to have about 1.5 times the oxidizing potential of chlorine with contact times for the anti-microbial action of ozone being typically four to five times less than that of chlorine.
Ozone has been shown to be a highly reactive oxidant that is capable of killing microorganisms such as bacteria as well as reacting with other chemicals such as pesticides and herbicides. Of course, a major advantage of ozone is its natural decomposition into oxygen and thus its use in disinfecting food products is highly beneficial as it decomposes into a non-toxic gas. It therefore does not impart odour to, or taint, food products and no residual compounds or toxic residue remain. Rinse water can be discharged to the environment or used for other applications without additional treatment or decontamination.
In prior art disinfecting processes known to applicant to use ozone, venturi injection systems and bubble diffusers have been used to mix ozone into water. In the case of venturi injectors, water is forced through a convergent conical body, initiating a pressure differential between the inlet and the outlet of the system. This creates a vacuum inside the body of the injector, thereby initiating a flow of ozone rich air through a suction port.
As regards bubble diffusers, ozone rich air is emitted in bubbles beneath the surface of the water. Irrespective of the problems further identified below, bubble diffusers suffer from an inherent disadvantage in that diffuser holes frequently become fouled over time thereby decreasing the efficiency of the system.
In both instances, ozone is dissolved into the water, typically from an ozone rich air, and an appreciable proportion of the sterilizing ability of the ozone may be spent in sterilizing the water itself. This leaves a reduced amount of ozone available for effective disinfecting of the ultimate target that may be fresh produce, for example.
Furthermore, these prior art systems appear to allow free gaseous ozone to be released into the atmosphere in higher concentrations than is permitted by regulatory standards. It is to be noted that free ozone in the air is harmful when it exceeds predetermined concentrations.
In this regard it is to be noted that in the European Union, the current target value for ozone concentrations is reported to be 120 μg/m3 which is about 60 nmol/mol. This target applies to all member states in accordance with Directive 2008/50/EC although there is no date set for formalizing this as a requirement and it is treated as a long-term objective. In the USA, in May 2008, the Environmental Protection Agency (EPA) lowered its ozone standard from 80 nmol/mol to 75 nmol/mol. This was done in spite of the fact that the Agency's own scientists and advisory board had recommended lowering the standard to 60 nmol/mol. The EPA has developed an Air Quality Index to help explain air pollution levels to the general public and presently the current standards describe an eight-hour average ozone mole fraction of 85 to 104 nmol/mol as “unhealthy for sensitive groups”; 105 nmol/mol to 124 nmol/mol as “unhealthy”; and 125 nmol/mol to 404 nmol/mol as “very unhealthy”. The World Health Organization recommends 51 nmol/mol.
Excess ozone in the air is therefore quite undesirable and it is important that any disinfecting device using ozone as its active disinfecting medium should not release any appreciable quantities of ozone into the atmosphere, whilst providing an effective concentration to destroy target bacteria etc.
In our earlier international patent application identified above, the proposal for sensing water flow through a mixer was to monitor the increase in pressure in the mixer when water was applied under pressure to the mixer. This expedient did not operate effectively and alternative controls needed to be investigated.